Burning oil Nissan won't honor warranty

Hi my Nissan Titan is burning oil at 50,000 miles. Nissan won't honor the warranty. I have oil change receipts but they say that there was sludge in the engine and that that can only come from not changing the oil enough. But I did change it and have the receipts to prove it! Any ideas on how to get Nissan to honor the warranty? Does anyone know of this happening with other Titans or NVs? By the way, mine's a 2012.
Thank you! I look forward to hearing your replies.
Unhappy with Nissan